Based on the latest findings, scientists say they are on their way to fully sequencing the Neandertal genome, possibly as soon as two years from now.

Some experts in the field, however, remain skeptical.

Putting together a complete Neandertal genetic library could take decades, according to archaeologist John Shea, who teaches at the State University of New York in Stony Brook.

And "accumulating a library is a first step," said Shea, who was not involved in the research released today.

"We need to know how to read the books, i.e., to know what differences in particular genes mean for growth and behavior."

But scientists are a long way from having a sufficient number of samples to conclusively test ancestry, he says.

DNA breaks down during fossilization, which also complicates analysis. What's more, Neandertal bones are typically soft and easily contaminated by fixatives or excessive handling once they are excavated.

"There are vastly more contaminated specimens" than pure ones, Shea said. "If you find that a Neandertal fossil has modern human DNA, is it mixed lineage or contamination?"

Anthropologist Erik Trinkaus, who was involved with last month's interbreeding study, says that today's DNA reports are consistent with findings based on non-DNA fossil analysis.

"All of this only says that the DNA analysis is probably close to correct, if not new in its conclusions," said the scientist, who is based at Washington University in St. Louis, Missouri.

"In other words, their work so far is solely a test of whether they can do it, what we call a pilot study." Going further may be a challenge.

"They have one individual and are unlikely to get very many more, given the poor organic preservation of most Pleistocene [1.8 million years ago to 11,500 years ago] fossil humans," Trinkaus said.